ACTG 338 is a Phase II comparative trial of 3 antiretroviral combinations for stable HIV-1 infected children on therapy. The three treatment arms (ZVD+3TC, D4T+Ritonavir, and ZDV+D4T+Ritonavir) will be evaluated with respect to change in plasma viral load from baseline to 48 weeks, and for safety and tolerance. ACTG 338 is the first study of a Phase II Master Rolling Arm Protocol (PRAM), designed to study new therapeutic arms as "rolling screens" through multiple generations of PRAM studies.